1. Field of the Invention
The present invention relates generally to placement of cutting elements on a rotary drag bit for use in drilling subterranean formations and, more specifically, to placement on various regions of the bit body of certain types of superabrasive cutting elements specifically engineered to better accommodate certain types of loading experienced in those regions during drilling.
2. State of the Art
Superabrasive, also termed "superhard", materials such as diamond and cubic boron nitride are employed in cutting elements for many commercial applications. One major industrial application where synthetic diamond structures are commonly employed is in cutting elements on drill bits for oil and gas drilling.
Polycrystalline diamond compact cutting elements, commonly known as PDC's, have been commercially available in planar geometries for over 20 years. PDC's may be self-supporting or may comprise a substantially planar diamond table bonded during formation to a supporting substrate. A diamond table/substrate cutting element structure is formed by stacking into a cell layers of fine diamond crystals (100 microns or less) and metal catalyst powder, alternating with wafer-like metal substrates of cemented tungsten carbide or other suitable materials. In some cases, the catalyst material may be incorporated in the substrate in addition to or in lieu of using a powder catalyst intermixed with the diamond crystals. A loaded receptacle is subsequently placed in an ultrahigh temperature (typically 1450-1600.degree. C.), ultrahigh pressure (typically 50-70 kilobar) diamond press, wherein the diamond crystals, stimulated by the catalytic effect of the metal power, bond to each other and to the substrate material. The spaces in the diamond table between the diamond to diamond bonds are filled with residual metal catalysis. A so-called thermally stable PDC product (commonly termed a "TSP") may be formed by leaching out the metal in the diamond table. Alternatively, silicon, which possesses a coefficient of thermal expansion similar to that of diamond, may be used to bond diamond particles to produce an Si-bonded TSP. TSP's are capable of enduring higher temperatures (on the order of 1200.degree. C.) without degradation in comparison to normal PDC's, which experience thermal degradation upon exposure to temperatures of about 750-800.degree. C.
While PDC and TSP cutting elements employed in rotary drag bits for earth boring have achieved major advances in obtainable rate of penetration (ROP) while drilling and in greatly expanding the types of formations suitable for drilling with diamond bits at economically viable cost, the diamond table/substrate configurations of state of the art PDC planar cutting elements leave something to be desired from a stressrelated structural standpoint due to internal residual stresses induced during fabrication. TSP's, which are generally formed as free-standing structures without a substrate or backing, have fewer manufacturing-induced internal stresses, but the internal structure of certain types of TSP's renders them somewhat brittle, and certain techniques by which they may be affixed to a bit crown may induce stresses.
To elaborate on the foregoing, one undesirable aspect of PDC cutting elements which contributes to their less than optimum performance under loading during drilling involves the residual stresses in the diamond table and in the supporting WC substrate, which stresses are induced during the manufacturing process as the cutting elements are returned to ambient temperature and pressure. While the diamond table is generally in compression and the substrate in tension, state of the art planar cutting elements exhibit a continuous area of undesirable residual tensile stress at or near the diamond and WC interface at the periphery of the cutting element and another ring of tensile stress on the cutting face just radially inward of its periphery.
As a result of the diamond table/substrate interface-area tensile stresses, PDC cutting elements are susceptible to spalling and delamination of the diamond table from the substrate due to loading from Normal, or axial, forces generated along the bit axis by the drill string, which is the dominant loading at the center (cone) and nose of a typical rotary drag bit.
As a result of the cutting face residual tensile stresses in the diamond table, bending attributable to the tangential or torsional loading of the cutting element by the formation primarily attributable to bit rotation may cause fracture of the diamond table. It is believed that such degradation of the cutting element is due at least in part to lack of sufficient stiffness of the cutting element so that, when encountering the formation, the diamond table actually flexes due to lack of sufficient rigidity or stiffness. As diamond has an extremely low strain rate to failure, only a small amount of flex can initiate fracture. This type of loading is generally dominant at the flank and shoulder of a typical rotary drag bit.
TSP cutting elements, as noted above, suffer fewer undesirable residual stresses as a result of the fabrication process since they are not bonded to a substrate, but the leached types of such cutting elements in particular are less impact-resistant than PDC's due to the porous nature of the diamond table. Moreover, it has been known in the art to bond TSP's to supporting substrates or carrier elements, such as by brazing, which process can and does induce stresses in the diamond table and along the diamond/carrier interface. Further, it is known to coat leached TSP's with single- and multi-layer metal coatings (as taught, respectively, by U.S. Pat. Nos. 4,943,488 and 5,049,164) so that they might be metallurgically bonded to a bit matrix during the furnacing operation rather than merely mechanically retained in the matrix, offering greater security with greater exposure of diamond volume for cutting purposes. Such coating and bonding to the bit matrix also can and does induce stress in the diamond. Thus, even with TSP cutting elements, residual stresses present in the diamond volume may weaken the cutting element against drilling-induced stresses.
Analysis of cutting elements from used bits shows that about eighty-five percent (85%) of PDC cutting elements fail in fracture due to operational loads in combination with residual manufacturing process-induced stresses. Thus, a serious problem exists with state-of-the-art planar PDC cutting elements.
It has also been ascertained, both empirically and through finite element analysis (FEA) numerical modelling techniques, that stress-related failure of PDC and TSP cutting elements occurs nonuniformly over the face of any given bit, even when all of the cutting elements on the bit are identical and similarly back-raked and side-raked. It has been demonstrated that differences in bit cross-sectional profile, rock type, rock stresses, and filtration, as well as other parameters relating to cutting element placement and orientation, may each contribute to some extent to the state and magnitude of stresses experienced by an individual cutting element. Thus, in many instances, loading of cutting elements in closely adjacent positions on the bit body is vastly different in both type and degree.
While differing bit profiles and radial location of a given cutting element result in different magnitudes, types and locations of high-stress areas on a bit crown (all other conditions being equal), such high-stress areas and their characteristics can be predicted with reasonable certainty using FEA.
In general, it has been discovered by the inventors that high stresses attributable to high tangential or torsional loading are experienced on cutting elements located at the bit flank and shoulder, which may be defined as the transitional regions between the bit nose and the bit gage. With some bit profiles, the greatest tangential loading may be on the shoulder immediately below the gage (given a normal bit orientation of a downwardly-facing bit face) as the profile turns radially inwardly on the bit face. Other profiles may concentrate the loading on the flank farther below and radially inward of the gage. It appears, in any case, that the highest tangential or torsional loading occurs on the radially outermost side of the bit body profile.
In the same vein, it has been discovered that higher combined axial (Normal) and tangential loading with substantial axial and tangential components, dominated by axial loading, is experienced at the center and nose of the bit face.
Therefore, cutting elements located in the different regions of the bit face experience vastly different loading. The effects of the loading have been accommodated in state of the art bits by variations in back rake of the cutting elements and in redundancy in certain critical regions. However, as the real or "effective" back rake of a cutter may be, and usually is, different from the fixed back rake with respect to the bit axis, obtaining a beneficial back rake for damage control purposes may result in poor cutting action.
Each cutting element or "cutter" located at a given radius on a bit crown will traverse through a helical path upon each revolution of the bit. The geometry (pitch) of the helical path is determined by the rate of penetration of the bit (ROP) and the rotational speed of the bit. Mathematically, it can be shown that the helical angle relative to the horizontal (or a plane Normal to the bit axis) decreases from the center of the bit to the shoulder for a given ROP and rotary speed. Essentially, the innermost 11/2" to 2" of bit face radius centered about the bit axis experiences the greatest change in helix angle, going from near 90.degree. at the center to about 7.degree. at the 2" radius. The change in helix angle from that location to the bit gage is relatively small. This phenomenon of variance in "effective rake" of a cutter with radial location, bit rotational speed and ROP is known in the art, and a more detailed discussion thereof may be found in U.S. Pat. No. 5,377,773, assigned to the assignee of the present invention and incorporated herein by this reference.
Planar state of the art PDC's (and planar TSP's) are set at a given back rake (usually negative) on the bit face to enhance their ability to withstand axial loading, which is dominated by the weight on bit (WOB). By comparing the effective back rake of a cutter (taking into account the helix angle for a given ROP and rotary bit speed), it is easy to see that cutters in the innermost 0" to 2" of radius from the bit axis or centerline have effective back rakes which are very high in comparison to those in other positions on the bit crown.
High back rakes have been shown to have the ability to carry much higher relative axial loads. It is known that the highest individual loading on cutters occurs from the center to the nose of the bit. This is a result of the substantial or even dominant axial component of the combined axial and tangential loading on a cutter in that region, and in the single cutter coverage for a given radius necessitated by the limited bit face area at and surrounding the center of the bit. Current PDC bit design thus dictates that cutter back rake be varied from high negative back rakes in the center to less negative back rakes toward the flank and shoulder. The higher center cutter negative back rakes provide more protection to the cutter against fracture damage by axial loading, the higher negative back rake beneficially orienting the tensile-stressed region at the diamond table/WC substrate interface against shear failure. Particularly high back rakes are further necessitated by the aforementioned high helix angle which produces a relatively more positive back rake, thus requiring more negative back rake to achieve a "net" negative back rake to avoid cutter damage.
While the higher effective negative back rake permits the use of conventional, state of the art planar PDC cutters in the center region, such higher effective back rakes reduce the aggressiveness of the cutter. This drawback becomes more critical to bit performance with distance from the center of the bit, high negative back rakes at the flank and shoulder to accommodate tangential or torsional-dominated loading on the cutters being very disadvantageous given the large volume of formation material to be cut at the larger diameters of those regions. Further, in bits with high design ROP or to which high WOB is applied, axial loads in the center of the bit may exceed the load-bearing capacity of standard cutters, even with high negative back rake.
Several approaches have been taken to cutting element design in order to accommodate operational stresses. For purposes of this application, such cutting elements will be referred to as "engineered" cutting elements. For example, U.S. patent application Ser. No. 08/164,481, filed Dec. 9, 1993, now U.S. Pat. No. 5,435,403 and assigned to the assignee of the present invention, discloses cutting elements engineered to better withstand bending stresses (resulting from tangential or torsional bit loading) by employing a transversely-extending, thickened portion of the superabrasive material table, or another transversely-extending reinforcing element proximate the interface between the superabrasive table and the supporting tungsten carbide (WC) substrate. This design, providing a "bar" of additional superabrasive material thickness, also offers more superabrasive volume for better durability against excessive wear. Also disclosed are preferred orientations and groupings of such cutting elements for maximum cutting effect, wear-resistance and stress-resistance.
U.S. patent application Ser. No. 08/353,453, filed Dec. 9, 1994 and also assigned to the assignee of the present invention, discloses further structural improvements to accommodate bending stresses on cutting elements, such as a rearwardly-extending strut of superabrasive material oriented transversely with respect to the superabrasive material table of a cutting element.
The disclosure of each of the referenced '481 and '453 applications is incorporated herein by this reference.
A so-called "sawtooth" planar PDC cutting element, developed by General Electric and having a series of concentric, planar or sawtooth cross-section rings at the PDC diamond table WC substrate interface has been demonstrated to withstand higher axial loading via reduction and redistribution of diamond table and table/substrate interface tensile stresses. This results in a strengthened cutting element in both tangential and Normal (axial) loading directions, but is most valuable in preventing damage from axial loading of the bit by providing a non-planar diamond table/substrate interface. The symmetrical structure of the diamond table/substrate interface is also advantageous, as not requiring a specific, preferential rotational orientation of a sawtooth cutting element on the bit face, unlike some other cutting element designs which employ parallel interface ridges extending across the cutting element.
Yet another recent cutting element engineering improvement is disclosed in U.S. patent application Ser. No. 08/039,858, filed Mar. 30, 1993 and assigned to the assignee of the present invention, and incorporated herein by this reference. This application discloses and claims use of a tapered or flared substrate which enhances the robustness of the cutting element in certain high compressive strength formations by providing superior support to the diamond table against loading experienced when the bit is first employed, particularly before normal wear flats form on the cutting elements. The tapered or flared substrate provides an effectively stiffer backing to the diamond table against tangential loading and an enlarged surface area adjacent the cutting edge to accommodate a portion of the Normal or axial loading.
Still another notable improvement in cutting element design is disclosed and claimed in U.S. patent application Ser. No. 07/893,704, filed Jun. 5, 1992, assigned to the assignee of the present invention, and incorporated herein by this reference. This application discloses and claims the use of multiple chamfers at the periphery of a PDC cutting face, which geometry enhances the resistance of the cutting element to impactinduced fracture. Moreover, if the angle of the outermost chamfer is substantially matched to the effective back rake of the cutting element, a bearing surface is provided to reduce the loading per unit area on the side of the diamond table, thus enhancing resistance to axial or Normal forces experienced by the cutting element.
Even with the aforementioned advances in cutter design, there has been little or no recognition in the art prior to the present invention that bit profile design and cutter design, placement and orientation on a bit crown should be approached from a "global" standpoint for optimum results of ROP and robust structural characteristics. Specifically, the art has not recognized the importance of understanding each cutter on a bit crown as a load-bearing structure, taking into account the residual stresses present in the cutter, mechanical loading (axial, tangential and the resultant combined axial/tangential loading), thermal loading during the drilling operation due to cutting friction and limitations or constraints in heat transfer from the diamond table, wear or abrasion of the cutters, available material choices, and bit profile and cutter geometry as well as rock strength and other formation characteristics.
Given the recognition of the importance of these factors by the inventors and the ability to design and select cutter type, placement and orientation, it has been realized by the inventors that, while it might be possible to employ engineered cutting elements of only one type over the entire face of a bit, the accommodation of the cutting element design to the complex and different loads applied on different regions of the bit face would not be optimized.
It has also been ascertained by the inventors that selective placement of specific types of engineered cutting elements on rotary drag bits in certain regions, in combination with conventional cutting elements, may result in more robust bits with a longer effective life and higher potential ROP, the engineered cutting elements accommodating the high- or complex-stress loading and complementing the conventional cutting elements. In other words, it is possible, but not preferred, to employ a combination of engineering and conventional cutting elements in accordance with the present invention.